Aircraft thrust augmentation systems are known which duct portions of flight engine thrust into the wings of the aircraft for discharge onto or adjacent to the wing flaps. These augmentation systems provide a steady state flow of engine exhaust during at least portions of the aircraft's flight, takeoff or landing phases to increase airflow over the flaps and thereby increase wing lift during periods when the aircraft's speed is below or approaching wing stall speed. Many of these systems use diverted axial engine fan or nozzle flow through dedicated ducting constructed in the main flight wings of the aircraft. Some of these systems provide individual thrust supply lines to each flap.
Disadvantages of these systems include the weight and wing structural load penalties from the ducting or supply lines. Also, failure of a propulsion engine, for example during takeoff, can result in loss of not only propulsion from the main engine, but loss of the additional thrust to a specific wing or set of flaps, which can cause a disproportionate lift generated by one of the wings and further exacerbates the loss of engine thrust. Plumbing for large amounts of relatively high pressure/temperature gas requires increased duct weight (to resist pressure) and increased diameter (which results in increased weight plus loss of aircraft volume) to accommodate insulation which prevents heat from soaking into the vehicle. Ducting also is often required to run the length of the wing(s) to reach each flap, which not only incurs large pressure losses but also reduces the resulting fuel holding capability of the wing(s). Each of the ducts or dedicated lines also requires individual controls to open or shut the duct or line, which further adds complexity and cost.
These systems have limited ability to throttle the augmented flow to the flaps, resulting in the inability to vary the wing lift during aircraft acceleration for takeoff or prior to landing. The limited ability to throttle occurs when exit area for the mass flow is choked (Mach number is 1) for a given design mass flow (pressure ratio). Mass flows below this value cause the system to un-choke. An un-choked system allows backflow and subsequent non-uniform flow which in turn can cause asymmetric lift. Augmentation flow therefore varies only with main engine thrust, which can be out of phase with the need for augmentation flow. For example, during preparation for landing the main flight engines are throttled back at the same time when the flaps are extended to increase wing lift, therefore airflow over the flaps from the augmentation system is not optimized when it is most needed.